This invention relates to an optical fiber gyro in which any phase difference .DELTA..theta. can be detected with high sensitivity.
In an optical fiber gyro, a laser beam is passed both clockwise and counterclockwise through an optical fiber loop which is fabricated by winding an optical fiber a number of turns, so that the two light beams are subjected to interference and the phase difference .DELTA..theta. due to the Sagnac effect is detected. The phase difference .DELTA..theta. is proportional to the angular velocity .OMEGA. of the optical fiber gyro: ##EQU1## The angular velocity .OMEGA. can be calculated by detected .DELTA..theta.. In the above-described equation, N is the number of turns of the optical fiber loop, A is the sectional area of the loop, C is the optical velocity in a vacuum, and .lambda. is the wavelength of a light source.
An optical fiber gyro utilizing the Sagnac effect is considerably effective as a means for determining the angular velocity of a rotating object.
A variety of optical fiber gyroscopes with different modulation method employed have been proposed in the art.
A fundamental system in which no modulation is employed and both light beams are allowed to propagate in one and the same optical path is advantageous in that no remaining optical path difference is caused. However, in the case of the non-modulated system, the output signal of the optical detector includes a phase difference .DELTA..theta. in the following form: EQU I (t).varies.(1+cos (.DELTA..theta.)) (2)
Since it is a cosine function, when .DELTA..theta. is close to zero (0) the variation is small and the sensitivity is low.
In order to overcome this drawback, a method has been employed in which the optical path is divided into two optical paths, and a .pi./2 phase shifter is inserted in one of the two optical paths.
FIG. 4 is an explanatory diagram showing the arrangement of a conventional optical fiber gyro.
In FIG. 4, reference numeral 31 designates a laser; and 32, 33, 34 and 35, half-mirrors which are disposed at the four corners of a square in a manner such that their surfaces are in parallel with the diagonal lines of the square.
Further in FIG. 4, reference numerals 36 and 37 designate condenser lenses which are adapted to concentrate the laser beam and to apply the laser beam thus concentrated to the two ends of an optical fiber loop 38.
The optical fiber loop is a single mode fiber or a polarization maintaining fiber.
The coherent beam from the laser 31 is divided into two light beams by the half-mirror 32. One of the two light beams, namely, a clockwise light beam passes through the half-mirror 33 and reaches the end face of the fiber through the lens 36, thus passing through the optical fiber loop 38 clockwise. The light beam is then applied to an optical detector 39 after being reflected by the half-mirrors 35 and 34.
The other light beam, namely, a counterclosewise light beam is reflected by the half-mirror 32 and is applied to the optical fiber through the half-mirror 35 and the lens 37. This light beam passes through the optical fiber loop while turning counterclockwise as many times as the number of turns of the loop, and is applied to the optical detector via the half-mirror 33 and the half-mirror 34.
If the half-mirrors 32 through 35 are disposed exactly at the four corners of the square and the two light beams go along the sides of the square, the optical path of the clockwise beam is equal to that of the counterclockwise beam. The phase difference .DELTA..theta. to the rotation of the optical fiber loop at an angular velocity .OMEGA. is divided into two parts, and the wavefunction f.sub.1 (t) of the clockwise beam at the optical detector and the wavefunction f.sub.2 (t) of the counterclockwise beam are represented by the following expressions (3) and (4), respectively: EQU f.sub.1 (t)=E.sub.0 e.sup.i(.omega.t+.DELTA..theta./ 2) (3) EQU f.sub.2 (t)=E.sub.0 e.sup.i(.omega.t-.DELTA..theta./ 2) (4)
where E.sub.0 is the amplitude. The same amplitude E.sub.0 is found in the expression of the two beams, because the amplitudes of the two beams are equal in the ideal case.
The optical detector outputs the square of the sum of the input. That is, the output I.sub.1 (t) is: EQU I.sub.1 (t)=2 E.sub.0.sup.2 (1+cos .DELTA..theta.) (5)
This corresponds to expression (2). In order to eliminate the cosine function, the following method is employed:
As shown in FIG. 4, a phase shifter 40 for shifting the phase by .pi./2 is inserted in one of the optical paths.
Therefore, instead of the wavefunction of expression (4), the wavefunction f.sub.3 (t) of the following expression (6) is given to the counterclockwise light beam: EQU f.sub.3 (t)=E.sub.0 e.sup.i(.omega.t+.pi. /2-.DELTA..theta./2) (6)
The optical detector detects the square of the sum of f.sub.1 and f.sub.3, and therefore the output I.sub.2 is: EQU I.sub.2 (t)=2 E.sub.0.sup.2 (1+sin .DELTA..theta.) (7)
The output I.sub.2 (t) is high in sensitivity when .DELTA..theta. is close to zero (0), and it is low in sensitivity when .DELTA..theta. is close to .+-..pi./2.
As is apparent from the above description, expressions (5) and (7) are advantageous in one aspect, but disadvantageous in another aspect. However, it may be said that they are complementary to each other. Expressions (5) and (7) should thus be selectively employed according to the data .DELTA..theta..
However, in practice, it is considerably difficult to insert the phase shifter 40 in the light path, and to remove it therefrom. More specifically, if the phase shifter is moved, it is considerably difficult to set the phase shifter at its original position and in the original direction. This may result in an error.